What is return value optimization?

Question

Reading through the answer for Split a string in C++?, here, a comment says

This should be split(s, delim, elems); return elems;, to enable return value optimization.

The relevant piece of code is

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, elems);
    return elems;
}

Line 3 and 4 were earlier

 return split(s, delim, elems);

Can you please explain what benefit do we get by splitting that line of code?